Very cold arctic air sank into the Northern Plains in mid January 2009. The light winds of 5 to 15 mph at times caused wind chills to run 40 to 60 below zero across much of eastern North Dakota. Wind chills fell to 40 to 50 below zero Tuesday evening into Wednesday. The coldest wind chills of 40 to 60 below zero occurred Wednesday (14th) night into Thursday (15th) morning. More 40 to 50 below wind chills occurred off and on Thursday evening through Friday (16th) morning.